1. Field of the Invention
The present invention relates to an electrophotographic toner used for visualizing an electrostatic charge latent image formed by an electrophotographic method, an electrostatic printing method, an electrostatic recording method, or the like, and an image-forming system and method using the electrophotographic toner.
2. Background Art
For example, in an electrophotographic method, a recording image is formed as follows. A photoconductive light-sensitive material (hereinafter referred to as “photoconductor”) is electrostatically charged and exposed to light, so that an electrostatic charge latent image is formed on the photoconductor. Then, the electrostatic charge latent image is developed by a fine particle-like toner containing a colorant, or the like, contained in a resin as a binder. The obtained toner image is transferred onto a sheet of recording paper. Then, the toner image is fixed on the sheet of recording paper to thereby form a recording image. In the process of forming such a recording image, development of the electrostatic charge latent image by the fine particle-like toner and fixation of the toner image on the sheet of recording paper are particularly important steps.
Heretofore, a magnetic-brush developing method using a two-component developing agent made of a toner and a magnetic carrier is used widely as a method of developing an electrostatic charge latent image by a toner. A heat-roller fixing method high in thermal efficiency to make high-speed fixing possible, and a fixing method using a film or the like for improving thermal response characteristic have been put into practical use as a method of fixing the toner on a sheet of recording paper.
In the case of use of such a fixing method, development of a toner capable of being fixed in the condition that the temperature and pressure of the heat roller are reduced to a lower temperature and a lower pressure by reduction in consumed electric power of a fixing heater and a drive motor has been demanded from the following viewpoints of:
(1) shortening the warm-up time unit until fixing can be allowed after the fixing unit starts;
(2) preventing fixing failure due to heat absorption of the sheet of recording paper to thereby make it possible to keep image quality good in continuous paper supply;
(3) preventing the sheet of recording paper from being curled or burned due to overheating; and
(4) reducing load applied on the heat roller to thereby simplify the structure of the fixing unit and reduce the size of the fixing unit.
On the other hand, in a recent image-forming system, a laser beam printer using a laser beam for exposing a photoconductive light-sensitive material to light in order to reproduce a recording image by dots on the basis of a modulating signal given by a computer's instruction has been developed with the advance of development of information apparatuses. Particularly, the recent laser beam printer needs to generate an image with higher quality. The size of the laser beam needs to be narrowed and reduced. The dot density needs to be increased in a range of from 600 to 1200 dpi (dots/inch). The particle size of the carrier also needs to be reduced. Therefore, use of a small-particle-size toner having a volume-average particle size of not larger than 10 μm and a small-particle-size carrier having a weight-average particle size of not larger than 100 μm has been advanced.
The cost of such a small-particle-size toner, however, becomes high because the yield in pulverizing and classifying steps is lowered at the time of production of the toner. For this reason, it is generally difficult to put a toner smaller than 4 μm into practical use. Generally, while a fine-particle toner is cut so that the mean particle size of the toner is selected to be in a range of from 4 μm to 10 μm, external additives to be added to the toner are used and the recipe for external additives are improved to improve fluidity of the toner.
On the other hand, with the advance of reduction in particle size of the toner, the particle size of the carrier is reduced to a weight-average particle size of not larger than 100 μm to increase the specific surface area of the carrier to thereby improve characteristic of frictional charge between the carrier and the toner. If the particle size of the carrier is smaller than 30 μm, the magnetic force of the carrier is however reduced so that the toner is easily deposited on the electrostatic charge image holding member because of electrostatic suction force. Therefore, the carrier is classified by particle size to obtain a mean particle size range of from 30 μm to 100 μm and surfaces of the carrier particles are coated with a resin as occasion demands.
With the improvement of the particle size distribution and the improvement of fluidity and charging characteristic, the small-particle-size toner and the developing agent have been put into practical use in an image-forming system such as a copying machine or a printer. A problem peculiar to the small-particle-size toner, however, occurs when printing is actually performed by the system and particularly when printing is repeated by 10 pages or more per minute. Reduction in life of the developing agent due to the carrier spent by the toner and reduction in life of the photoconductor due to the photoconductor filmed with the toner occur easily.
With respect to the fixation of the small-particle-size toner, it is difficult to obtain a good image fixing strength because a great deal of energy is required for obtaining a certain fixing strength compared with the fixation of a large-particle-size toner. Particularly in the fixing step, it is necessary to increase the temperature and pressure of the heat roller. Hence, there is a problem that high reliability and simplification of the fixing unit and reduction in size and cost of the fixing unit can hardly be attained. Accordingly, improvement in fixing characteristic of the toner has been strongly demanded in order to solve this problem.
Addition of wax into the fixing resin to improve the fixing characteristic of the toner has been al ready known. For example, this technique has been disclosed in JP-A-52-3304, JP-A-52-3305, JP-A-57-52574, etc. Such waxes are added for the purpose of preventing the toner from being deposited on the heat roller at a low temperature or at a high temperature, that is, preventing a so-called offset phenomenon.
For example, JP-A-5-313413 has disclosed addition of an ethylene- or propylene-α-olefin copolymer having a viscosity of not higher than 10000 poises at 140° C. into a vinyl-based copolymer having a specific molecular weight distribution in order to improve low-temperature fixing characteristic, anti-offset characteristic and non-agglomeration characteristic of the toner.
For the same purpose, JP-A-7-287413 has disclosed addition of paraffin wax having an absorption calorie peak (melting point) of 75° C. to 85° C. measured by a differential scanning calorimeter (DSC), JP-A-8-314181, JP-A-9-179335 and JP-A-9-319139 have disclosed addition of natural gas-based Fischer-Tropsch wax having a melting point of 85° C. to 110° C. measured by a DSC, JP-A-6-324513 has disclosed addition of polyethylene wax having a melting point of 85° C. to 110° C. measured by a DSC, JP-A-7-36218 has disclosed addition of polyethylene-based wax prepared by removal of a component having a melting point of not higher than 50° C. by a distillation method or the like so as to have a melting point of 70° C. to 120° C. measured by a DSC, and JP-A-8-114942 has disclosed addition of polyethylene wax having a weight-average molecular weight (Mw) of lower than 1000.
On the other hand, fluidity, heat resistance, durability and storage stability of the toner are lowered when low-melting wax is added to the toner. To improve these characteristics, JP-A-6-123994 has disclosed use of wax having a weight-average molecular weight/number-average molecular weight ratio (Mw/Mn) of not higher than 1.5, JP-A-7-209909 has disclosed use of ethylene-based olefin polymer wax having a melting viscosity of 0.5 mPa·s to 10 mPa·s at 140° C. and a rate of penetration of not higher than 3.0 dmm, JP-A-7-287418 has disclosed use of Fischer-Tropsch wax having an average molecular weight of not lower than 1000, and JP-A-10-104875 has disclosed use of a combination of paraffin wax and Fischer-Tropsch wax exhibiting a predetermined viscosity.
Although the fixing characteristic of the toner can be improved by these related-art techniques, the actual situation is that a toner more improved in low-temperature fixing characteristic has been required from the point of view of increase in speed or reduction in energy of a printer. Moreover, more durability against rubbing than that in the related art has been required in recent years because an image is rubbed due to repeated passage of a sheet of recording paper through the system in double-sided printing, multiplex printing, scale-down edition, or the like. Moreover, an electrophotographic developing agent having higher durability against reduction in running cost, reduction in number of maintenance, etc. has been required.
An object of the invention is to provide an electrophotographic toner satisfying the aforementioned needs.
Specifically, a problem that the invention is to solve is to provide a toner in which energy required for fixing the toner is so low that the temperature and pressure of a heat roller can be reduced when a heat-roller fixing method is used, in which an offset phenomenon hardly occurs, which has both high peeling strength and high rubbing strength at a low temperature and which is good in fluidity, heat resistance, durability and storage stability. More specifically, it is to provide an electrophotographic toner in which reduction in life of a developing agent due to a carrier spent by the toner hardly occurs and in which reduction in life of a photoconductor due to the photoconductor filmed with the toner hardly occurs.
Another object of the invention is to provide a stable image-forming system and method using the electrophotographic toner.